The invention relates generally to ultrasound ranging, and more particularly to systems and methods for performing ultrasound ranging in the presence of ultrasound interference.
Ultrasound ranging is a technique for computing the distance between two ultrasound transducers. The principle of ultrasound ranging is illustrated in FIG. 1, which shows two ultrasound transducers 10,20 separated by a distance. One of the ultrasound transducers is designated as a transmitting transducer 10 and the other is designated as a receiving transducer 20. To measure the distance between the transducers 10,20, the transmitting transducer 10 transmits an ultrasound pulse 25, which is detected by the receiving transducer 20. The distance, d, between the transducers 10,20 is computed as
d=vxcfx84
where v is the velocity of the ultrasound pulse 25 in the medium between the transducers 10,20 and xcfx84 is the time of flight of the ultrasound pulse 25 in traveling from the transmitting transducer 10 to the receiving transducer 20.
One application of ultrasound ranging is in ultrasound positional tracking to track the position of a device within a three-dimensional (3-D) coordinate system. Referring to FIG. 2, this is accomplished by mounting one or more ranging transducers 110 on the device 115 being tracked and providing four or more reference transducers 120-1 to 120-4 that are spaced apart. In this particular example, the device 115 being tracked is a catheter tip. The ranging transducer 110 acts as a receiving transducer and each of the reference transducers 120-1 to 120-4 can act both as a receiving and transmitting transducer.
To establish the 3-D coordinate system, the reference transducers 120-1 to 120-4 are sequentially excited to transmit ultrasound pulses (not shown). When each reference transducer 120-1 to 120-4 transmits an ultrasound pulse, the other reference transducers 120-1 to 120-4 detect the ultrasound pulse. The relative distances between the reference transducers 120-1 to 120-4 are then computed by performing ultrasound ranging on each of the detected ultrasound pulses. The computed distances are then triangulated to determine the relative positions between the reference transducers 120-1 to 120-4 in 3-D space. The relative positions between the reference transducers 120-1 to 120-4 are then mapped onto the 3-D coordinate system to provide a reference for tracking the position of the ranging transducer 110 in the 3-D coordinate system.
To track the position of the ranging transducer 110, and hence the device 115 carrying the ranging transducer 110, in the 3-D coordinate system, the reference transducers 120-1 to 120-4 are sequentially excited to transmit ultrasound pulses. When each of the reference transducers 120-1 to 120-4 transmits an ultrasound pulse, the ranging transducer 110 detects the ultrasound pulse. The distance d1-d4 between the ranging transducer 110 and each of the reference transducers 120-1 to 120-4 is computed by performing ultrasound ranging on each of the detected ultrasound pulses. The computed distances are then triangulated to determine the relative position of the ranging transducer 110 to the reference transducers 120-1 to 120-4 in 3-D space. The position of the ranging transducer 110 in the 3-D coordinate system is then determined based on the relative position of the ranging transducer 110 to the reference transducers 120-1 to 120-4 and the known positions of reference transducers 120-1 to 120-4 in the 3-D coordinate system.
An example of a tracking system using ultrasound ranging is the Realtime Position Management(trademark) (RPM) tracking system developed commercially by Cardiac Pathways Corporation, now part of Boston Scientific Corp. The RPM system uses ultrasound ranging to track the positions of medical devices, including reference catheters, mapping catheters and ablation catheters.
Because ultrasound ranging relies on the transmission and detection of ultrasound pulses to measure distance, it is vulnerable to ultrasound interference from ultrasound sources, e.g., an ultrasound imager. For example, ultrasound interference may be detected by the receiving transducer 20 and misinterpreted as an ultrasound pulse from the transmitting transducer 10, producing an erroneous distance measurement.
Therefore, there is need for systems and methods that enable the use of ultrasound ranging in the presence of ultrasound interference.
The present inventions are directed to systems and methods that enable the use of ultrasound measuring equipment in the presence of ultrasound interference.
In accordance with a first aspect of the present inventions, a distance measuring system comprises first and second transducers, and an ultrasound ranging subsystem coupled to the first and second transducers for performing a plurality of distance measurements between the first and second transducers. By way of non-limiting example, the distance measuring system, in performing the distance measurements, comprises a pulse generator coupled to the first transducer for generating and transmitting transmit pulses to the first transducer, a threshold detector coupled to the second transducer for detecting receive pulses from the second detector, and measurement means (e.g., a digital counter) coupled to the pulse generator and the threshold detector. In this case, for each distance measurement, the measurement means triggers the pulse generator to generate and transmit a transmit pulse to the first transducer, measures the elapsed time between transmission of the transmit pulse and detection of a receive pulse by the threshold detector, and generates the distance measurement based on the measured elapsed time.
The distance measuring system further comprises a filter coupled to the ultrasound ranging subsystem for filtering ultrasound interference from the plurality of distance measurements (such as, e.g., eight), and outputting a distance based on the filtered distance measurements. The distance measurement system can have various applications, including medical applications, in which case, the first and second transducers can be mounted on a catheter.
In the preferred embodiment, the filter filters the ultrasound interference by selecting one of the plurality distance measurements, in which case, the outputted distance is the selected distance measurement. Because the ultrasound interference will typically represent itself as the shortest distance measurement, the selected distance measurement is preferably greater than the minimum distance measurement (such as, e.g., the maximum distance measurement), thereby filtering the ultrasound interference out.
Although the present inventions should not be so limited in its broadest aspects, the filter sequentially receives the distance measurements, and filters the ultrasound interference from the last N distance measurements. In this case, the filter may filter the last N distance measurements by selecting one of them. So that the system is more responsive to movements of the transducers, the filter can compute a distance variation of the N distance measurements, and compare the distance variation to a threshold value. The filter can then output the distance when the distance variation exceeds the threshold value, while outputting the most recent distance measurement received from the ultrasound ranging subsystem otherwise. In effect, the filtering is only accomplished when there is ultrasound interference, thereby providing more responsiveness to the distance measuring process. The distance variation computation can be accomplished in a variety of ways, including taking the difference between the maximum and minimum of the last N distance measurements, calculating the variance of the last N distance measurements, or calculating the second derivative of the last N distance measurements.
In accordance with a second aspect of the present inventions, a method for measuring the distance between two transducers comprises performing a plurality of distance measurements (e.g., eight) between the transducers. For example, the distance measurement can comprise exciting one of the transducers to transmit an ultrasound pulse, and measuring the time for the ultrasound pulse to reach the other transducer.
The method further comprises filtering ultrasound interference from the plurality of distance measurements, and outputting a distance based on the filtered distance measurements. The distance measurements can be filtered by, e.g., selecting one of the plurality distance measurements, in which case, the outputted distance will be the selected distance measurement. The selected distance measurement is preferably more than the minimum distance measurement, such as, e.g., the maximum distance measurement.
Although the present inventions should not be some limited in its broadest aspects, the distance measurements are sequentially received, and the ultrasound interference is filtered from the last N distance measurements. In this case, the last N distance measurements can be filtered by selecting one of them. So that the method is more responsive to movements of the transducers, a distance variation of the N distance measurements can be computed and compared to a threshold value. The distance can then be outputted when the distance variation exceeds the threshold value, while the most recent distance measurement received from the ultrasound ranging subsystem can be outputted otherwise, thereby providing for a more responsive method. The distance variation computation can be accomplished in a variety of ways, including taking the difference between the maximum and minimum of the last N distance measurements, calculating the variance of the last N distance measurements, or calculating the second derivative of the last N distance measurements.